Semiconductor device MESFET with upper and lower layers

ABSTRACT

This invention is for improving the radiation hardness or radiation resistance of GaAs MESFETs. According to this invention, an n-type active layer is formed by doping in the GaAs crystal. The n-type active layer includes an upper layer and a heavy doped lower layer. A Schottky gate electrode is provided on the active layer so that the carrier concentration in the active layer and the thickness of the active layer are set to required values. According to this invention, not only in the case of a total dose of exposure radiation of R=1×10 9  roentgens but also in the case of a higher total dose, at least one of the threshold voltage V th  of the GaAs MESFET, the saturated drain current I dss  there of, or the transconductance g m  will remain in their tolerable ranges. Consequently a semiconductor device comprising the GaAs MESFET and a signal processing circuit cooperatively combined therewith can operate normally as initially designed, with the result of conspicuously improved radiation hardness.

RELATED APPLICATIONS

This is a continuation-in-part of our copending application Ser. No. 07/409,600 filed in Sept. 19, 1989, now abandoned for a Semiconductor Device.

DETAILED DESCRIPTION OF THE INVENTION

1. Background of the Invention

This invention relates to a semiconductor device using a GaAs metal-semiconductor field effect transistor (MESFET), specifically to that which can be used in machines and instruments requiring radiation hardness or radiation resistance.

2. Related Background Art

The devices which are used in aerospace systems and near nuclear furnaces are required to have high radiation hardness. The radiation includes gamma (γ) rays, neutron rays, proton rays, etc. Generally the gallium arsenide (GaAs) MESFETs and ICs based upon these FETs will withstand a total exposure dose of 1×10⁸ roentgens with little if any change in characteristics. By contrast, silicon (Si) MOS circuits have failed at doses of 1×10⁶ roentgens (Proceedings of Symposium of Space Development, 1987, ps. 35 to 38).

For improving the radiation hardness of the GaAs MESFET, the following devices have been proposed. In a first one, a p-type layer is buried below an n-type active layer to thereby decrease leakage current to the substrate, and the threshold voltage of a GaAs FET is improving in the radiation hardness. In a second one, the Schottky gate of a GaAs FET is shortened.

But these prior art devices have improved the radiation hardness up to a total exposure dose R of about 1×10⁸ roentgens, but it cannot be said that these prior art devices have succeeded in attaining the sufficiently practical level (1×10⁹ roentgens). Under these circumstances, no practical transistor having a radiation hardness level of about 1×10⁹ roentgens has been realized.

On the other hand, Physics of Semiconductor Device, 2nd Edition, S.M. Sze., P. 334, describes a MESFET having two active layers. This MESFET has realized superior characteristics since the carrier concentration in the surface of the active layer lowers, and consequently the MESFET is free from the affects of a deletion layer.

Therefore an object of this invention is to provide a GaAs MESFET which has a simple structure and a high radiation hardness which has been improved by especially making use of changes of at least one of the threshold voltage, the saturated drain current, and the transconductance.

SUMMARY OF THE INVENTION

The inventors have noticed that when radiation is applied to a GaAs MESFET, the change amount ΔV_(th) of the threshold voltage V_(th) in the saturation region, the change rate α=I_(dssA) /I_(dss) of saturated drain current at normal gate bias I_(dss), and the change rate β=g_(mA) /g_(m) of a transconductance g_(m) has a constant relationship with the effective thickness t_(a) =t_(a1) +t_(a2) of the active layer and the change amount ΔN_(D) of the carrier concentrations N_(1D), N_(2D) and has found that the change amount ΔN_(D) has a constant quantitative relationship with the total exposure dose R. Based on this finding, they have completed this invention.

That is, this invention relates to a semiconductor device including a MESFET having an active layer doped in a GaAs crystal and a threshold voltage V_(th), the active layer comprising an upper layer having a carrier concentration N_(1D) and an effective thickness t_(a1), and a lower layer having a carrier concentration N_(2D) and an effective thickness t_(a2) (N_(1D<N) _(2D)); and which normally operates when at least one of three conditions that a change amount ΔV_(th) of the threshold voltage V_(th) of the MESFET is within a tolerable change amount ΔV_(thL), a change rate α of the saturated drain current I_(dss) is within a tolerable change rate α_(L), and a change rate β of the transconductance g_(m) is a tolerable change rate β_(L) are satisfied, when ΔN_(D) represents a decrease in the amount of the carrier concentrations N_(1D), N_(2D) of the active layer due to the radiation exposure of a total dose R equal to or higher than 1×10⁹ roentgens, μ and μ_(A) represent carrier mobilities in the active layer respectively before and after the radiation exposure, ε represents a dielectric constant of the active layer, and q represents an electron charge, at least one of the following three conditions

an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer is given by

    t.sub.a <{(2ε·ΔV.sub.thL)/(q·ΔN.sub.D)}.sup.1/2

and,

a carrier concentration N_(2D) of the lower layer of the active layer before the radiation exposure is given by

    N.sub.2D >ΔN.sub.D /{1-[α.sub.L (μ/μ.sub.A)].sup.1/2 }.

    N.sub.2D >ΔN.sub.D /{1-β.sub.L (μ/μ.sub.A)}.

In this arrangement, under irradiation of a total exposure dose even below 1×10⁹ roentgens, at least any one of the threshold voltage V_(th), the saturated drain current I_(dss), and the transconductance g_(m) are within their corresponding preset ranges (tolerable ranges determined by the signal processing circuit). Consequently, the semiconductor device according to this invention comprising the GaAs MESFET and a signal processing circuit can operate normally as initially designed.

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indication preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a GaAs MESFET explaining the principle of this invention;

FIG. 2 is a graph showing the relationship of the total exposure dose R and a change amount ΔV_(th) of the threshold voltage of the MESFET involved in this invention;

FIG. 3 is a graph showing the relationship of the total exposure dose R and a decrease amount ΔN_(D) of the carrier concentration;

FIG. 4 is a graph showing the result of an experiment concerning the relation of the total exposure dose R and a change amount ΔV_(th) of the threshold voltage of a conventional MESFET;

FIG. 5 is a graph showing the relationship of the total exposure dose R dependence of a change rate α of the saturated drain current of the MESFET involved in this invention;

FIG. 6 is a graph showing the relationship of the total exposure dose R and a change of the saturated drain current I_(dss) ;

FIG. 7 is a graph showing the relationship of the total exposure dose R and a change rate β of the transconductance of the MESFET involved in this invention;

FIG. 8 is a graph showing the results of experiments on the relationship of the total exposure dose R and a change of the transconductance g_(m) of the conventional MESFET;

FIGS. 9 to 11 are characteristic curves of the threshold voltage, saturated drain current, and transconductance of the MESFET involved in this invention and of the conventional FETs for comparison in radiation hardness; and

FIG. 12 is a sectional view of a GaAs MESFET having an active layer in one layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention will be explained in good detail with reference to the drawings showing the principle and structure of this invention.

The semiconductor device according to this invention comprises a GaAs MESFET, and a signal processing circuit cooperatively combined with the MESFET. The MESFET and the signal processing circuit can provide combination circuits, e.g., amplifiers, inverters, oscillators, digital logic arrays, etc.

FIG. 1 shows a sectional view of a GaAs MESFET having a recess gate structure. As shown in FIG. 1, an n-type active layer 2, and a heavily doped n-type (n⁺ -type) contact region 3 are formed on a semi-insulating GaAs substrate 1. The active layer 2 includes an upper layer 21 of a thickness t_(a1) and a lower layer 22 of a thickness t_(a2). Parts of the n-type GaAs active layer 2 and the n⁺ -type contact region 3 for a gate region to be provided are etched off to form a recess structure. Then a source electrode 4 and a drain electrode 5 of ohmic metal are formed on the n⁺ -type contact region 3 by the vacuum evaporation. A gate electrode 6 of Schottky metal is formed on the n-type active layer 2. The part of the n-type active layer 2 directly below the gate electrode 6 has a sufficiently smaller thickness, compared with the n-type active layers of the conventional MESFETs, and the active layer 2 has a higher carrier concentration N_(2D), compared with the carrier concentrations of the conventional MESFETs.

The MESFET in this combination circuit involved in this invention has a preset threshold voltage V_(th), a saturated drain current I_(dss) at normal gate bias, and a transconductance g_(m) in the saturation region. It has been known that their values change under radiation exposure. When a changed threshold voltage V_(thA), a changed saturated drain current I_(dssA), and a changed transconductance g_(mA) to which their initial values have changed due to radiation exposure are out of their preset ranges by the signal processing circuit, this combination circuit does not normally operate. Hereinafter in this specification, a tolerable value of a change amount ΔV_(th) of the threshold voltage V_(th) is represented by a tolerable change amount ΔV_(thL). A tolerable value of a change rate α=I_(dssA) /I_(dss) of the saturated drain current I_(dss) is represented by a tolerable change rate α_(L), and a tolerable value of a change rate α=g_(mA) /g_(m) of the transconductance is represented by a tolerable change rate β_(L). The values of ΔV_(thL), α_(L) and β_(L) vary depending on designs of the above described signal processing circuit, but generally ΔV_(thL) is equal to or lower than 0.2 V, and α_(L) and β_(L) are equal to or higher than 0.8.

As described above, it is known that the threshold voltage V_(th), etc. change under radiation exposure. Causes for these changes have been reported, firstly, decreases in a carrier concentration of the active layer due to radiation exposure, and secondly decreases in an electron mobility therein due to radiation exposure. The inventors discussed the first cause in detail and found the relationship

    ΔN.sub.D =b·R.sup.c                         (1)

where b and c are constants holds between a decreased amount ΔN_(D) of a carrier concentrations N_(1D), N_(2D) and a total exposure dose R. Formula 1 holds when an initial carrier concentration N_(2D) (before radiation exposure) of the active layer is 1×10¹⁷ to 1×10¹⁹ cm⁻³, a total dose R of exposure radiation is 1×10⁸ to 1×10¹⁰ roentgens. The constants b and c have some ranges depending on changes of an initial carrier concentration of the active layer, qualities of the substrates, etc.

The values of the above-described constants b, c were given by the following two methods. In a first method, some samples of the GaAs MESFETs were prepared, and those samples were exposed to radiation to measure the change amounts ΔV_(th) of the threshold voltages. That is, since the relationship of Formula 5 which will be described below is given between the change amount ΔV_(th) and the carrier concentration decrease amount ΔN_(D), the relationship between the total dose R and the carrier concentration decrease amount ΔN_(D) can be given empirically by measuring the total dose R of radiation and the change amount ΔV_(th). Based on this relationship, the constants b, c in the above-described Formula 1 could be given.

According to the experiments conducted by the inventors, the constants b and c have ranges of

    1.99×10.sup.10 ≦b≦3.98×10.sup.10

    0.5≦c≦0.8

and the typical values are b=3.06×10¹⁰, c=0.678. Therefore, the typical value of the decreased amounts ΔN_(D) of the carrier concentration is defined by

    ΔN.sub.D =3.06×10.sup.10 ·R.sup.0.678.

This first method is for measuring a change amount ΔV_(th) of the threshold voltage to calculate a carrier concentration decrease amount ΔN_(D) and accordingly for indirectly giving the relationship between the carrier concentration decrease amount ΔN_(D) and the total dose R. In contrast to the first method, in a second method, the relationship between the carrier concentration decrease amount ΔN_(D) and the total dose R is directly given. To this end, the Hall effect was measured.

First, a GaAs Hall element which has been made n-type by Si doping was prepared. The n-type GaAs layer was formed by epitaxial growth, and the carrier concentration distribution in the n-type GaAs layer is constant in the directions of the depth, length and width of the Hall element. A plurality of such samples having a 100 Å to 2 μm-thickness n-type GaAs layer and a 1×10¹⁶ cm⁻³ to 5×10¹⁸ cm⁻³ -carrier concentration were prepared. The Hall effect was measured on these samples to give the carrier concentration and Hall mobility. Then when a carrier concentration is represented by N, and a carrier concentration change amount is indicated by ΔN, a carrier extinction number is given by

    ΔN=N(before radiation)-N(after radiation).

According to these experiments, the constants b, c have an allowance of

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3.

The constant b is represented by b=6.65×10⁵, and the constant c is represented by c=1.17. Therefore, the representative value of the carrier concentration decrease amount ΔN_(D) is

    ΔN.sub.D =6.65×10.sup.5 ·R.sup.1.17.

As seen from the above, the first indirect measuring method and the second direct measuring method give different values of the constants b, c. The reason for the occurrence of such differences is presumed as follows. That is, in the indirect measurement in accordance with the first method, on the assumption of the following (a), (b) and (c), the carrier concentration decrease amount ΔN_(D) was calculated based on the actually measured values of the change amount ΔV_(th).

(a) The depth profile of the effective carrier concentration is constant over the active layer of the GaAs MESFET.

(b) The decrease in the effective carrier concentration occurs uniformly over the active layer, and the thickness remains unchanged for whole irradiations.

(c) The carrier mobility is not changed by γ-ray radiation.

In contrast to this, in the direct measure in accordance with the second method, the carrier concentration decrease amount ΔN_(D) was measured actually by the Hall effect, and the above-described assumption was not used. It is considered that due to this, the difference in the values of the constants b, c between the first and the second methods took place. But anyway it is doubtless that the above-described Formula 1 is given. In the following description, the result of the second method, i.e., the measurement of the Hall effect, is taken as the values of the constants b, c.

The result of the measurement of the Hall effect is expressed in a two-dimensional logarithmic graph where γ-ray exposure dose is on the horizontal axis, and the carrier extinction number is on the vertical axis as shown by the dots in FIG. 3. The above-described Formula 1 expressed by

    ΔN.sub.D =6.65×10.sup.5 ·R.sup.1.17

as described above is plotted as indicated by the dotted line in FIG. 3.

The carrier concentration decrease amount ΔN_(D) can be given by the above-described Hall effect measurement, but the general relationship between the change amount ΔV_(th) of the threshold voltage V_(th) of the GaAs MESFET and the carrier concentration decrease amount ΔN_(D) can be given theoretically as follows.

The theoretical value of the threshold voltage V_(th) of this GaAs MESFET is given by ##EQU1## given by solving the one dimensional Poisson equation

    d.sup.2 φ.sub.1 /dχ.sup.2 =-qN.sub.1D /ε

    (0≦χ≦t.sub.a1)

    d.sup.2 φ.sub.2 /dχ.sup.2 =-qN.sub.2D /ε

    (t.sub.a1 ≦χ≦t.sub.a1 +t.sub.a2)

under the boundary conditions of

    dφ/dχ=0 at χ=t.sub.a1 +d(d>t.sub.a2)

    φ=-(V.sub.bi -V.sub.G) at χ=0.

In Formula 2, V_(bi) represents a built-in voltage of the MESFET; q, an electron charge; and ε, a dielectric constant of the n-type active layer 2. When the carrier concentrations N_(1D), N_(2D) of the n-type active layer 2 becomes N_(1DA), N_(2DA) due to radiation exposure, a threshold voltage V_(thA) after the radiation exposure is given by ##EQU2## Then, a change of the threshold voltage V_(th) due to the radiation exposure is given by ##EQU3## In Formula 4 ##EQU4## Therefore, when a decrease in the carrier concentration due to radiation exposure is given by

    ΔN.sub.D =N.sub.D -N.sub.DA,

    with t.sub.a =t.sub.a1 +t.sub.a2,

    ΔV.sub.th ={(q·t.sub.a.sup.2)/2ε}·ΔN.sub.D(5).

In this Formula 5, the carrier concentration decreased amount ΔN_(D) can be given empirically by measuring Hall effect on the samples (Hall elements). Accordingly the theoretical value of the change amount ΔV_(th) of the threshold voltage can be given. Reversely, the change amount ΔV_(th) can be given experimentally using samples of the MESFET.

The inventors further studied the change amount ΔV_(th) of the threshold voltage V_(th) by irradiation by gamma rays in total exposure doses R=1×10⁸ roentgens, 1×10⁹ roentgens and 3×10⁹ roentgens to a MESFET of FIG. 1 having the active layer 2 of a thickness of 450=(150+300) Å. The result is shown in FIG. 2 by black points.

Then, the above-described relationship given by measuring the Hall effect,

    ΔN.sub.D =6.65×10.sup.5 ·R.sup.1.17

is adapted to the MESFET of FIG. 1 to give the change amount ΔV_(th) of the threshold voltage V_(th) using the above-described Formula 5. The result is as indicated by the dotted line in FIG. 2. The theoretical value of the change amount ΔV_(th) of the threshold voltage well agrees with the experimental value thereof.

In FIG. 2, with a total dose of R=1×10⁹ roentgens, a change amount ΔV_(th) of the threshold voltage is as low as about 0.04 V. Therefore, it is confirmed that the radiation hardness is conspicuously improved when the active layer 2 has a thickness of about 450 Å.

Formula 1 described above was derived from actually measured values for six total doses of R=1×10⁸ roentgens, 3×10⁸ roentgens, 6×10⁸ reoengens, 1×10⁹ roentgens, 2×10⁹ roentgens and 3×10⁹ roentgens. It can be said that these values are insufficient data to derive a general formula. Then the inventors conducted a further experiment of irradiation by gamma rays from cobalt 60 to the GaAs MESFET of FIG. 12 which had the same geometrical structure except that in the active layer as the GaAs MESFET involved in this invention and has the active layer 2 in one layer. The active layer 2 had an effective thickness t_(a) of 1130 Å so that the carrier concentration N_(D) is 2.09×10¹⁷ cm³ . In this experiment, total exposure doses were R=1×10⁶ roentgens, 1×10⁷ roentgens, 1×10⁸ roentgens, 3×10⁸ roentgens, 1×10⁹ roentgens, 2×10⁹ roentgens and 3×10⁹ roentgens. The resultant change amounts ΔV_(th) of the threshold voltage are shown in FIG. 4 by the black points. These points well agree with the theoretical values indicated by the dotted line. The change amounts ΔV_(th) after a radiation exposure of R=1×10⁹ roentgens, however, was about 0.4 V, which was remarkably inferior to that in FIG. 2.

The results of these experiments on the threshold voltage show the following. Firstly, a major cause for the degradation of the threshold voltage of the MESFET due to radiation damage is a decrease in the carrier concentration of the active layer, and it was found that Formula 1 well expresses a decrease in the carrier concentration under the radiation exposure. Secondly, it has found that the change amount ΔV_(th) of the threshold voltage V_(th) can be set at a required value by setting only the thickness t_(a`=t) _(a1) +t_(a2) of the active layer. Specifically, as shown in FIG. 4, when the thickness t_(a) of the active layer 2 is set at about 1000 Å as in the conventional MESFETs, the radiation hardness is insufficient, but as shown in FIG. 2, when the thickness t_(a) =t_(a1) +t_(a2) is set at 450 Å, the radiation hardness is conspicuously improved.

Then, the inventors actually measured changes of the saturated drain current I_(dss) due to the radiation exposure of the same GaAs MESFETs as those which exhibited the threshold voltages V_(th) of FIGS. 2 and 4. As a result, the characteristic of the change rate α of the saturated drain current I_(dss) of FIG. 5 was obtained in use of same MESFET of FIG. 2. The change rate α of the saturated drain current I_(dss) of FIG. 6 was obtained in use of same MESFET of FIG. 4. In FIGS. 5 and 6, the black points indicate the experimental values, and the dotted lines indicate the theoretical values given by applying Formula 10 described below to Formula 1.

The theoretical formula for the change rate α=I_(dssA) /I_(dss) of the saturated drain current I_(dss) will be derived below. The saturated drain current I_(dss) of the MESFET is given for an intrinsic FET with a source resistance R_(s) kept out of consideration, by ##EQU5## In Formula 6, W_(g) represents a gate width; μ, an electron mobility in the active layer 2; L_(g), a gate length; and V_(G), a gate voltage. In Formula 6, N_(1D) /N_(2D) =0 based on N_(1D) <N_(2D). Therefore, Formula 6 is rewritten d₁ =t_(a1). When a saturated drain current I_(dss) for V_(G) =V_(bi) is represented by I_(DSS) for simplifying the computation, Formula 6 is rewritten into

    I.sub.DSS ={(W.sub.G ·μ·q.sup.2 ·N.sub.2D)/(ε·L.sub.g)}·[t.sub.a1 ·t.sub.a2.sup.2 +(t.sub.a2 -t.sub.a1)t.sub.a2.sup.2 /2 -t.sub.a2.sup.3 /3]                                       (7)

when a saturated drain current after the radiation exposure is represented by I_(DSSA), a change rate α due to the radiation exposure is given based on Formula 7 by ##EQU6## where N_(2DA) is a carrier concentration of the lower layer after the radiation exposure and is given by

    N.sub.2DA =N.sub.2D -ΔN.sub.D                        (9).

Then Formula 8 is substituted by Formula 9 into

    α{μ.sub.A (N.sub.2D -ΔN.sub.D).sup.2 }/(Δ·N.sub.2D.sup.2)                       (10).

Then Formula 10 will be discussed below. It is found that the change rate α is influenced by changes (μ⊖μ_(A)) of the electron mobility μ due to the radiation exposure. But μ_(A) /μ is around 0.95-0.98 when the carrier concentration before the radiation exposure is about 1×10¹⁸ cm⁻³. The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with μ_(A) /μ=0.95. The results are shown by the dotted lines of FIGS. 5 and 6. As described above, it was confirmed that the results agreed with the experimental values.

These experiments on the saturated drain current I_(dss) and the studies of their results show the following. Firstly, a major cause for the degradation of the I_(dss) of the MESFET as total dose radiation affects is decreases of in the carrier concentration of the active layer, and it was found that Formula 1 is very descriptive of the decrease in the carrier concentration due to the radiation exposure. Secondly, the change rate α of the saturated drain current can be set at a required value by setting only the initial carrier concentration (before the radiation exposure) N_(2D) of the lower layer 22 of the active layer, because μ is a constant in Formula 10, the value of μ_(A) /μ can be approximated, and ΔN_(D) can be determined depending on a total radiation dose in Formula 1. Specifically, when the carrier concentration N_(D) of the active layer 2 is set at about 2×10¹⁷ cm⁻³ as in the conventional ones, the radiation hardness is insufficient as seen in FIG. 6. When the carrier concentration N_(2D) of the lower layer is set at 1×10¹⁸ cm⁻³, the radiation hardness is outstandingly improved as seen in FIG. 5.

Then, the inventors actually measured changes of the transconductances g_(m) due to the radiation exposure in the saturation regions of the same GaAs MESFETs as those which exhibited the threshold voltages V_(th) of FIGS. 2 and 4 and the saturated drain current I_(dss) characteristics of FIGS. 5 and 6. As results, the change rate β of the transconductance g_(m) of FIG. 7 was obtained in use of same MESFET of FIG. 2 and 5, and the change rate β of the transconductance g_(m) of FIG. 8 was obtained in use of same MESFET of FIG. 4 and 6. In FIGS. 7 and 8, the black points indicate the experimental values, and the dotted lines indicate the theoretical values given by applying Formula 15 described below to Formula 1.

The theoretical formula for the change rate β=g_(mA) /g_(m) of the transconductance will be derived below. A transconductance g_(m) in the saturation region of the MESFET is given for an intrinsic FET with a source resistance R_(s) kept out of consideration, by ##EQU7## In this Formula,

    d.sub.1 ={t.sub.a1.sup.2 -N.sub.1D ·t.sub.a1.sup.2 /N.sub.2D +2ε·(V.sub.bi -V.sub.G)/(q·N.sub.2D)}.sup.1/2(11).

In Formula 11, N_(1D) /N_(2D) =0 based on N_(1D) <N_(2D). Therefore, Formula 11 is rewritten d₁ =t_(a1). When a transconductance g_(m) for V_(G) =V_(bi) is represented by g_(mmax) for simplifying the computation, Formula 11 is rewritten into ##EQU8##

A change rate β due to the radiation exposure is given based on Formula 12 by ##EQU9## where N_(2DA) is a carrier concentration of the lower layer after the radiation exposure and given by

    N.sub.2DA =N.sub.2D -ΔN.sub.D                        (14).

Then Formula 13 is substituted by Formula 14 into

    β={μ.sub.A (N.sub.2D -ΔN.sub.D)}/(μ·N.sub.2D)(15).

Then Formula 15 will be discussed below. It is found that the change rate β is influenced by changes (μ→μ_(A)) of the electron mobility μ due to the radiation exposure. But μ_(A) /μ is around 0.95 when the carrier concentration before the radiation exposure is about 1×10¹⁸ cm⁻³ . The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with μ_(A) /μ=0.95. The results were the dot lines of FIGS. 7 and 8. As described above, it was confirmed that the results agreed with the experimental values.

These experiments and the studies of their results show the following. Firstly, a major cause for the degradation of the transconductance of the MESFET as a total dose radiation affect is decreases of the carrier concentration in the active layer, and it was found that Formula 1 is very descriptive of the decrease of the carrier concentration due to the radiation exposure. Secondly, the change rate β of the transconductance can be set at a required value by setting only the initial carrier concentration N_(2D) of the lower layer 22 of the active layer, because μ is a constant in Formula 15, the value of μ_(A) /μ can be approximated, and ΔN_(D) is determined based on Formula 1, depending on a radiation dose. Specifically, when the carrier concentration N_(D) of the active layer 2 is set at about 2×10¹⁷ cm⁻³ as in the conventional ones, the radiation hardness is insufficient as seen in FIG. 8. When the carrier concentration N_(2D) in the lower layer is set at 1×10¹⁸ cm⁻³, the radiation hardness is outstandingly improved as seen in FIG. 7.

It is possible that, based on the above described findings, a structure of a semiconductor device which is able to operate normally under the radiation exposure of a total dose R not only below 1×10⁹ roentgens but also a total dose R above 1×10⁹ roentgens can be specified based on a thickness t_(a) of the active layer 2 and a carrier concentration N_(2D) in the lower layer 22. That is, in order that a GaAs MESFET and a signal processing circuit are combined into such semiconductor device, and the signal processing circuit can operate as designed when a tolerable change amount of the threshold voltage V_(th) of the MESFET is ΔV_(thL), an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer 2 must be

    t.sub.a <{(2ε·ΔV.sub.thL)/(q·ΔN.sub.D)}.sup.1/2                                                        (16),

based on Formula 5. In this case, the carrier concentration N_(2D) of the lower layer 22 of the active layer 2 is given as follows based on Formula 2 by

    N.sub.2D ={[2ε/(q·t.sub.a.sup.2)]·(V.sub.bi -V.sub.th)}                                               (17)

Here, when a tolerable change amount ΔV_(thL) of the threshold voltage V_(th) for a total exposure dose of R=1×10⁹ roentgens is specifically computed by

    ΔV.sub.thL =0.1V(ΔV.sub.th <0.1V),

a change amount ΔN_(D) of the carrier concentration is given by

    ΔN.sub.D =2.25×10.sup.16 cm.sup.-3.

An effective thickness t_(a) =t_(a1) +t_(a2) of the active layer 2 is given below 529 Å based on Formula 16. Further, with a thickness of the active layer 2 set at below 529 Å, the carrier concentrations N_(1D), N_(2D) of the active layer is determined by their interrelationships with the thickness t_(a1), t_(a2). In this case, a dielectric constant of the active layer ##EQU10## an electron charge

    q=1.602×10.sup.-19 C,

and a built-in voltage V_(bi) =0.7V.

In order that the combination circuit which relates to the semiconductor device operates as required when a tolerable change rate of the saturated drain current I_(dss) of the MESFET is represented by α_(L), an initial carrier concentration N_(2D) of the lower layer 22 of the active layer 2 must be, based on Formula 10 by

    N.sub.2D >ΔN.sub.D /{1-[α.sub.L ( μ/μ.sub.A)].sup.1/2 }(18).

In this case, an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer 2 is given by

    t.sub.a ={[2ε/(q·N.sub.2D)](V.sub.bi -V.sub.th)}(19).

Here, for a total exposure dose of R=1×10⁹ roentgens, with α_(L) (a tolerable change rate of the saturated drain current I_(DSS))=0.9 (I_(DSSA) >0.9I_(DSS)), a change amount ΔN_(D) of the carrier concentration is given based on Formula 1 by

    ΔN.sub.D =3.25×10.sup.16 cm.sup.-3,

and the carrier concentration N_(2D) of the lower layer 22 of the active layer 2 becomes above 8.44×10¹⁷ cm⁻³ based on Formula 10.

Further, in order that the GaAs MESFET and a signal processing circuit are combined into this semiconductor device, and the signal processing circuit can operate as designed when a tolerable change rate of the transconductance g_(m) in the saturation region of the MESFET is β_(L), an initial carrier concentration N_(2D) of the lower layer 22 of the active layer 2 must be, based on Formula 15,

    N.sub.2D >ΔN.sub.D /{1-β.sub.L (μ/μ.sub.A)}(21).

In this case, an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer 2 must be

    t.sub.a ={[ε/(q·N.sub.2D ](V.sub.bi -V.sub.th)}.sup.1/2(22).

Here, for a total exposure dose of R=1×10⁹ roentgens, with β_(L) (a tolerable change rate of the transconductance g_(mmax))=0.9(g_(mmaxA) >0.9 g_(mmax)), a change amount ΔN_(D) is given by

    ΔN.sub.D =2.25×10.sup.16 cm.sup.-3

based on Formula 1. The carrier concentration N_(2D) of the lower layer of the active layer 2 is given above 4.50×10¹⁷ cm⁻³ by Formula 15.

The semiconductor device according to this invention and the conventional ones will be compared in radiation hardness in FIG. 9 to 11. FIG. 9 shows change amounts ΔV_(th) of the threshold voltage V_(th) due to the radiation exposure. FIG. 10 shows change rates α of the saturated drain current I_(dss). FIG. 11 shows change rates β of the transconductance g_(m). In FIGS. 9 to 11, curves (a), (b) and (c) show characteristics of the conventional commercial MESFETs. The curve (b) corresponds to the characteristics of FIGS. 4, 6 and 8 for the active layer 2 of a 1130 Å effective thickness t_(a) and a carrier concentration of 2.09×10¹⁷ cm⁻³. The curve (d) shows the characteristics of a conventional HEMT (high electron mobility transistor). As evident from FIG. 9, these conventional semiconductor devices have change amounts ΔV_(th) of the threshold voltage as high as 0.2 to 0.3 V for a total dose of R=1×10⁹ roentgens. The curve (e) in FIG. 9 shows the characteristic of a MESFET having a p-type layer buried below an n-type active layer for decreasing leakage current to the substrate, and the change amount ΔV_(th) is suppressed to about 0.12 V for R=1×10⁹ rentgens. In contrast to this, in the MESFET according to this invention having the active layer 2 of an effective thickness t_(a) =t_(a1) +t_(a2) of 450 Å (corresponding to the characteristic of FIG. 2), the change amount ΔV_(th) is suppressed to a value lower than 0.1 V even for R=1×10⁹ roentgens as indicated by the curve (f), and it is found that the radiation hardness is much improved. It is evident from FIGS. 10 and 11 that such improvement in the radiation hardness is also exhibited in the saturated drain current I_(dss) and the transconductance g_(m).

In this invention, even under the radiation exposure of a total exposure dose equal to or higher than R=1×10⁸ roentgens, the values of the threshold voltage V_(th), saturated drain current I_(dss) and transconductance g_(m) remain within their tolerable ranges. A GaAs MESFET which is acknowledged as superior in radiation hardness characteristics must have radiation hardness for a total exposure dose of about 1.4×10⁸ to 4.3×10⁹ roentgens. For this exposure dose, the absorbed dose of GaAs is totally 1×10⁸ to 3×10⁹ rad. (1 roentgens=0.7 rads. in GaAs). On the other hand, the tolerable range of the change (positive shift) amount ΔV_(th) of the threshold voltage V_(th) is 0.2 V, and the tolerable ranges of the change rates α=I_(dss), β=g_(mA) /g_(m) of the saturated drain current I_(dss) and the transconductance g_(m) are about 80%. Specifically, when the change amount ΔV_(th) =0.15 V with a total exposure dose R=1.5×10⁹ roentgens, it can be said that the GaAs MESFET has superior radiation hardness.

But what has to be noted here is that the above described tolerable change amount ΔV_(thL), and the tolerable change rates α_(L), β_(L) greatly vary depending on circuits combined with the GaAs MESFET. Specifically, one example is SCFL (Source coupled FET Logic) circuits which have low integrity but have enabled high speed operation. In theses circuits, the operation speed is substantially determined by a current flowing two transistors in the buffer stages. Accordingly, when values of the V_(th), I_(dss) and g_(m) vary due to a radiation exposure, the operation speed greatly varies. But the influence on the operation speed by the changes of values of V_(th), I_(dss) and g_(m) can be reduced by 1/3 to 1/4 by setting the values of resistors of the SCFL circuit at suitable values. Consequently, even in a SCFL circuit which allows for a change of the operation speed of only 10%, the tolerable change is 200 m/V for the threshold voltage V_(th) (V_(thL) =0.2 V), and the tolerable changes for the saturated drain current I_(dss) and the transconductance g_(m) are about 20% (α_(L) =0.8, β_(L) =0.8).

By contrast, another example is a memory cell for a memory IC which has high integrity on a semiconductor chip, the tolerable range for those changes are narrowed. Specifically, in this IC, a time in which one small memory cell charges and discharges the data lines occupies a large part of a total access time. Furthermore, each memory cell has transistors, resistors, etc. miniaturized for reducing power consumption. Consequently, the operation speed greatly varies depending on changes of the parameters. Specifically, in order to keep the change of the memory access time within 20%, the tolerable change amount of the threshold voltage V_(th) is only 50 mV ΔV_(thL) =0.05 V), and the tolerable change rates of the saturated drain current I_(dss) and the transconductance g_(m) are only 10% (α_(L) =0.9, β_(L) =0.9).

This invention is not limited to the above described embodiment and covers various modifications.

For example, the active layer is not necessarily formed by epitaxial growth but may be formed by ion implantation. The recess structure of FIG. 1 is not essential.

From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

We claim:
 1. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal and a threshold voltage V_(th), the active layer comprising an upper layer having a carrier concentration N_(1D) and an effective thickness t_(a1), and a lower layer having a carrier concentration N_(2D) and an effective thickness t_(a2) (N_(1D) <N_(2D)); and which normally operates when a change ΔV_(th) in the threshold voltage V_(th) is within a tolerable amount ΔV_(thL),an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer being

    t.sub.a <{(2ε·ΔV.sub.thL)/(q·ΔN.sub.D)}.sup.1/2

where ΔN_(D) represents a decrease in the carrier concentrations N_(1D), N_(2D) due to radiation exposure of a total exposure dose R equal to or higher than 1×10⁹ roentgens, ε represents a dielectric constant of the active layer, and q represents an electron charge.
 2. A semiconductor device according to claim 1, wherein the decrease ΔN_(D) in the carrier concentrations N_(1D), N_(2D) is given by

    ΔN.sub.D =b·R.sup.c

where b and c are constants.
 3. A semiconductor device according to claim 2, wherein the constants b and c are

    1.99×10.sup.10 ≦b≦3.98×10.sup.10

    0.5≦c≦0.8.


4. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal, the active layer comprising an upper layer having a carrier concentration N_(1D), and a lower layer having a carrier concentration N_(2D) (N_(1D) <N_(2D)); and which normally operates when a change rate α=I_(dssA) /I_(dss) of a saturated drain current I_(dss) of the MESFET, where I_(dssA) represents the value to which the saturated drain current I_(dss) has changed, is within a tolerable rate α_(L), the carrier concentration N_(2D) of the lower layer before radiation exposure is given by

    N.sub.2D >ΔN.sub.D /{1-[α.sub.L (μ/μ.sub.A)].sup.1/2 }

where ΔN_(D) represents a decrease in the carrier concentrations N_(1D), N_(2D) of the active layer due to radiation exposure of a total exposure dose R equal to or higher than 1×10⁹ roentgens, and μ and μ_(A) represent carrier mobilities in the active layer respectively before and after the radiation exposure.
 5. A semiconductor device according to claim 4, wherein a decrease ΔN_(D) in the carrier concentration is given by

    ΔN.sub.D =b·R.sup.c

where b and c are constants.
 6. A semiconductor device according to claim 5, wherein the constants b and c are

    1.99×10.sup.10 ≦b≦3.98×10.sup.10

    0.5≦c≦0.8.


7. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal, the active layer comprising an upper layer having a carrier concentration N_(1D), and a lower layer having a carrier concentration N2_(D) (N_(1D) <N_(2D)); and which normally operates when a change rate β=g_(mA) /g_(m) of a transconductance g_(m) in the saturation region of the MESFET where g_(mA) represents the value to which the transconductance g_(m) has changed, is within a tolerable rate β_(L),a carrier concentration N_(2D) of the lower layer of the active layer before radiation exposure is given by

    N.sub.2D >ΔN.sub.D /{1-β.sub.L (μ/μ.sub.A)}

where ΔN_(2D) represents a decrease in the carrier concentrations N_(1D), N_(2D) of the active layer due to radiation exposure of a total exposure dose R equal to or higher than 1×10⁹ roentgens, and μ and μ_(A) represent carrier mobilities in the active layer respectively before and after the radiation exposure.
 8. A semiconductor device according to claim 7, wherein a decrease ΔN_(D) in the carrier concentration is given by

    ΔN.sub.D =b·R.sub.c

where b and c are constants.
 9. A semiconductor device according to claim 8, wherein the constants b and c are

    1.99×10.sup.10 ≦b≦3.98×10.sup.10

    0.5≦c≦0.8.


10. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal and a threshold voltage V_(th), the active layer comprising an upper layer having a carrier concentration N_(1D) and an effective thickness t_(a1), and a lower layer having a carrier concentration N_(2D) and an effective thickness t_(a2) (N_(1D) <N_(2D)); and which normally operates when a change ΔV_(th) in the threshold voltage V_(th) is within a tolerable amount ΔV_(thL), and a change rate α=I_(dssA) /I_(dss) of a saturated drain current I_(dss) of the MESFET where I_(dssA) represents the value to which the saturated drain current I_(dss) has changed, is within a tolerable rate α_(L),where ΔN_(D) represents a decrease in the carrier concentrations N_(1D), N_(2D) of the active layer due to radiation exposure of a total exposure dose R equal to or higher than 1×10⁹ roentgens, μ and μ_(A) represent carrier mobilities in the active layer respectively before and after the radiation exposure, ε represents a dielectric constant of the active layer, and q represents an electron charge, an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer being

    t.sub.a <{(2ε·ΔV.sub.thL)/(q·ΔN.sub.D)}1/2

and, a carrier concentration N_(2D) of the lower layer of the active layer before the radiation exposure is given by

    N.sub.2D >N.sub.D /{1-[α.sub.L (μ/μ.sub.A)]1/2}.


11. A semiconductor device according to claim 10, wherein a decrease ΔN_(D) in the carrier concentration is given by

    N.sub.D =b·R.sup.c

where b and c are constants.
 12. A semiconductor device according to claim 11, wherein the constants b and c are

    1.99×10.sup.10 ≦b≦3.98×10.sup.10

0.5≦c≦0.8.
 13. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal and a threshold voltage V_(th), the active layer comprising an upper layer having a carrier concentration N_(1D) and an effective thickness t_(a1), and a lower layer having a carrier concentration N_(2D) and an effective thickness t_(a2) (N_(1D) <N_(2D)); and which normally operates when a change ΔV_(th) in the threshold voltage V_(th) is within a tolerable amount ΔV_(thL), and a change rate β=g_(mA) /g_(m) of a transconductance g_(m) in the saturation region of the MESFET where g_(mA) represents a transconductance to which the transconductance g_(m) has changed, is within a tolerable rate β_(L),where a decrease ΔN_(D) in the carrier concentration N_(1D), N_(2D) due to radiation exposure of a total exposure dose R equal to or higher than 1×10⁹ roentgens, μ and μ_(A) represent carrier mobilities in the active layer respectively before and after the radiation exposure, ε represents a dielectric constant of the active layer, and q represents an electron charge, an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer being

    t.sub.a <{(2ε·ΔV.sub.thL)/(q·ΔN.sub.d)}1/2,

and a carrier concentration N_(2D) of the lower layer of the active layer before the radiation exposure being

    N.sub.2D >ΔN.sub.D /{1-β.sub.L (μ/μ.sub.A)}.


14. A semiconductor device according to claim 13, wherein a decrease ΔN_(D) in the carrier concentration is given by

    ΔN.sub.D =b·R.sup.c

where b and c are constants.
 15. A semiconductor device according to claim 14, wherein the constants b and c are

    1.99×10.sup.10 ≦b≦3.98×10.sup.10

    0.5≦c≦0.8.


16. A semiconductor device including a MESFET having an active layer doped in a GaAs crystal, the active layer comprising an upper layer having a carrier concentration N_(1D), and a lower layer having a carrier concentration N_(2D) (N_(1D) <N_(2D)); and which normally operates when a change rate α=I_(dssA) /I_(dss) of a saturated drain current I_(dss) of the MESFET where I_(dssA) represents the value to which the saturated drain current I_(dss) has changed, is within a tolerable rate α_(L), and a change rate β=g_(mA) /g_(m) of a transconductance in the saturation region of the MESFET where g_(mA) represents the value to which the transconductance g_(m) has changed, is within a tolerable rate β_(L),a carrier concentration N_(2D) of the lower layer of the active layer before radiation exposure being

    N.sub.2D >ΔN.sub.D /{1-[α.sub.L (μ/μ.sub.A)].sup.178 },

    and

    N.sub.2D >ΔN.sub.D /{1-β.sub.L (μ/μ.sub.A)}

where ΔN_(D) represents a decrease in the carrier concentrations N_(1D), N_(2D) of the active layer due to radiation exposure of a total exposure dose R equal to or higher than 1×10⁹ roentgens, and μ and μ_(A) represent carrier mobilities in the active layer respectively before and after the radiation exposure.
 17. A semiconductor device according to claim 16, wherein a decrease ΔN_(D) in the carrier concentration is given by

    ΔN.sub.D =b·R.sup.c

where b and c are constants.
 18. A semiconductor device according to claim 17, wherein the constants b and c are

    1.99×10.sup.10 ≦b≦3.98×10.sup.10

    0.5≦c≦0.8.


19. A semiconductor device including a MESFET having an active layer doped in a GaAs crystal and a threshold voltage V_(th), the active layer comprising an upper layer having a carrier concentration N_(1D) and an effective thickness t_(a1), and a lower layer having a carrier concentration N_(2D) and an effective thickness t_(a2) (N_(1D) <N_(2D)); and which normally operates when a change ΔV_(th) in the threshold voltage V_(th) is within a tolerable amount ΔV_(thL), a change rate α=I_(dssA) /I_(dss) of a saturated drain current I_(dss) of the MESFET where I_(dssA) represents the value to which the saturated drain current I_(dss) has changed, is within a tolerable rate α_(L), and a change rate β=g_(mA) /g_(m) of a transconductance g_(m) in the saturation region of the MESFET where g_(mA) represents the value to which the transconductance g_(m) has changed, is within a tolerable rate β_(L),where ΔN_(D) represents a decrease in the carrier concentrations N_(1D), N_(2D) due to radiation exposure of a total exposure dose R equal to or higher than 1×10⁹ roentgens, μ and μ_(A) represent carrier mobilities in the active layer respectively before and after radiation exposure, ε represents a dielectric constant of the active layer, and q represents an electron charge, an effective thickness t_(a) =t_(a1) +t_(a2) of the active layer being

    t.sub.a <{(2ε·αV.sub.thL)/(q·ΔN.sub.D)}.sup.1/2,

and a carrier concentration N_(2D) of the lower layer of the active layer before the radiation exposure being

    N.sub.2D >ΔN.sub.D /{1-[α.sub.L (μ/μ.sub.A)].sup.1/2 }

and

    N.sub.2D >ΔN.sub.D /{1-β.sub.L (μ/μ.sub.A)}.


20. A semiconductor device according to claim 19, wherein a decrease ΔN_(D) in the carrier concentration is given by

    ΔN.sub.D =b·R.sup.c

where b and c are constants.
 21. A semiconductor device according to claim 20, wherein the constants b and c are

    1.99×10.sup.10 ≦b ≦3.98×10.sup.10

    0.5≦c≦0.8.


22. A semiconductor device according to claim 2, wherein the constants b and c are

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3.


23. A semiconductor device according to claim 5, wherein the constants b and c are

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3.


24. A semiconductor device according to claim 8, wherein the constants b and c are

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3.


25. A semiconductor device according to claim 11, wherein the constants b and c are

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3.


26. A semiconductor device according to claim 14, wherein the constants b and c are

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3.


27. A semiconductor device according to claim 17, wherein the constants b and c are

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3.


28. A semiconductor device according to claim 20, wherein the constants b and c are

    5×10.sup.5 ≦b≦1×10.sup.6

    1.0≦c≦1.3. 